When parts are produced by a molding process, the molded article frequently has residual material that is extruded at mold interfaces, which is referred as residual flash material or "flash." This flash must be removed in a finishing operation. One technique that can be used to remove flash is machining and/or hand trimming, which is expensive and time consuming. It is frequently possible for small articles to use a shot blasting operation where the shot impacts the relatively thinner flash and removes the flash without significant damage to the underlying article. See, e.g., U.S. Pat. No. 5,676,588, U.S. Pat. No. 4,648,214, and U.S. Pat. No. 4,312,156, incorporated by reference, for basic teachings on flash removal.
One approach to shot blasting for flash removal uses the property of many materials to become embrittled at low or cryogenic temperatures. For example, many rubbers and plastics become brittle at temperatures obtained from cooling by evaporating liquid nitrogen in the material surroundings. Since flash is conventionally very thin, the flash will be cooled to brittle temperatures before the body of the material so that the flash will be readily removed by shot blasting that does not damage the article.
In order to be effectively cooled and exposed to the shot, articles are conventionally placed in a rotating drum so that the articles are continuously picked up and dropped from rotating projections, or "flights", in the drum. It is conventional to place the rotating drum at an angle to enhance the tumbling action of the flights. The design of conventional drums causes some articles to aggregate in the downhill comer of the drum, with a resulting loss in deflashing of the articles.
The deflashing shot is expelled by a throw wheel that accelerates the shot through an opening in the housing that contains the rotating basket and into the rotating basket. Conventional throw wheels have an even number of vanes for accelerating the shot and are subject to harmonic vibrations which reduce the life of the equipment Further, the expended shot merely collects in the bottom of the housing, which can impede the rotation of the drum and which requires a large supply of shot.
A particular problem for operating deflashing apparatus at cryogenic temperatures is the design of adequate seals for closures and for rotating shafts that penetrate the cryogenic housing. Conventional closure seals are formed of materials that become brittle at the operating temperature of the cryogenic deflashing apparatus or that become stuck together through freezing of ambient water vapor. Seals for rotating shafts, and the like, also become brittle at cryogenic temperatures with a limited operating lifetime or excessive leakage of the cooling nitrogen.
Conventional cryogenic equipment is generally provided as a fixed device. With fixed devices, batches of manufactured must be diverted from the production line to the devices, unless many of these devices are provided integral with the production lines. It would be desirable to provide portable deflashing apparatus that can be readily moved when a production line is not in use.
These problems are addressed by the present invention and an improved deflashing apparatus is provided. Accordingly, it is an object of the present invention to provide a rotating drum that maintains the articles in a continuous movement in the drum for improved flash removal.
Another object of the present invention is to provide for the reuse of shot to reduce the quantity of stored shot that is required.
Still another object of the present invention is to improve the flow of shot to the throw wheel and to reduce vibration in the throw wheel.
One other object of the present invention is to provide improved seal designs for operation at cryogenic temperatures.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.